Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI

[Suren Baghdasaryan]

On Mon, Oct 23, 2023 at 9:36 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 23.10.23 14:03, David Hildenbrand wrote:
> > On 22.10.23 17:46, Peter Xu wrote:
> >> On Fri, Oct 20, 2023 at 07:16:19PM +0200, David Hildenbrand wrote:
> >>> These are rather the vibes I'm getting from Peter. "Why rename it, could
> >>> confuse people because the original patches are old", "Why exclude it if it
> >>> has been included in the original patches". Not the kind of reasoning I can
> >>> relate to when it comes to upstreaming some patches.
> >>
> >> You can't blame anyone if you misunderstood and biased the question.
> >>
> >> The first question is definitely valid, even until now.  You guys still
> >> prefer to rename it, which I'm totally fine with.
> >>
> >> The 2nd question is wrong from your interpretation.  That's not my point,
> >> at least not starting from a few replies already.  What I was asking for is
> >> why such page movement between mm is dangerous.  I don't think I get solid
> >> answers even until now.
> >>
> >> Noticing "memcg is missing" is not an argument for "cross-mm is dangerous",
> >> it's a review comment.  Suren can address that.
> >>
> >> You'll propose a new feature that may tag an mm is not an argument either,
> >> if it's not merged yet.  We can also address that depending on what it is,
> >> also on which lands earlier.
> >>
> >> It'll be good to discuss these details even in a single-mm support.  Anyone
> >> would like to add that can already refer to discussion in this thread.
> >>
> >> I hope I'm clear.
> >>
> >
> > I said everything I had to say, go read what I wrote.
>
> Re-read your message after flying over first couple of paragraphs
> previously a bit quick too quickly (can easily happen when I'm told that
> I misunderstand questions and read them in a "biased" way).
>
> I'll happy to discuss cross-mm support once we actually need it. I just
> don't see the need to spend any energy on that right now, without any
> users on the horizon.
>
> [(a) I didn't blame anybody, I said that I don't understand the
> reasoning. (b) I hope I made it clear that this is added complexity (and
> not just currently dangerous) and so far I haven't heard a compelling
> argument why we should do any of that or even spend our time discussing
> that. (c) I never used "memcg is missing" as an argument for "cross-mm
> is dangerous", all about added complexity without actual users. (d) "it
> easily shows that there are cases  where this will require extra work --
> without any current benefits" -- is IMHO a perfectly fine argument
> against complexity that currently nobody needs]

Thanks for the discussion, folks!
I think posting the single-mm first and then following up with
cross-mm and its test would help us move forward. That will provide
functionality that is needed today quickly without unnecessary
distractions and will give us more time to discuss cross-mm support.
Also we will be able to test single-mm in isolation and make it more
solid before moving onto cross-mm.
I'll try to post the next version sometime this week.
Thanks,
Suren.

>
> --
> Cheers,
>
> David / dhildenb
>